The overall purpose of this project is to help elucidate the relationship between structure and function of viral and ultimately cellular nucleoprotein complexes. The interaction between cellular expression and viral genome structure will also be analyzed, in both bacterial and animal virus systems. In addition to biochemical approaches, I plan to take advantage of the permeability of cells and virus particles to psoralen derivatives such as 4,5',8-trimethylpsoralen (trioxsalen). These components cross-link the two strands of DNA in the presence of long wavelength ultraviolet light and have been shown to be useful probes for the intracellular "nucleosome" structure of cellular chromatin and viral DNA-protein complexes. They also provide a powerful tool for an analysis of the mechanism of cellular recombination and repair. Because of both the specificity of psoralen compounds for accessible regions of DNA such as internucleosomal bridges, and the ability to study intracellular and viral structures in vivo without prior disruption of the biological system, this approach will allow a detailed evaluation of the significance of the results from in vitro studies of nucleosome structure, DNA replication, recombination and repair enzymes, and viral genome structure. The projects currently underway can be divided according to the approach and system employed: (1) The structure of virion and intracellular SV40 nucleoprotein complexes will be analyzed in order to study the relationship between nucleoprotein structure, the function of tumor antigen, and the regulation of transcription and replication, employing both in vivo cross-linking and in vitro replication systems. (2) The intracellular structure of procaryotic "chromatin" will be investigated with respect to its protection from photoaddition of trioxsalen derivatives. A comparison will be made between E. coli DNA, lambda prophage DNA and super-infecting lambda DNA. (3) Photochemical inactivation of several virus classes will be used as a probe for the relationship between structure and biological activity.